The invention is directed to a method of tuning an optical interference device comprising a plurality of waveguides transmitting co-propagating light waves. The tuning is done by changing the optical path length of at least two of the waveguides. In particular, the optical path length is changed by changing the refractive index of at least a portion of two of the waveguide lengths by illuminating the portions of the waveguide lengths with ultraviolet light. The two irradiated waveguides are designed to have different responses to the ultraviolet light which allows a simultaneous irradiation to produce relatively different changes in optical path length of the at least two waveguides.
As optical waveguide telecommunications has moved into higher capacity applications and those which involve multiplexed signals and signals which must be delivered to different nodes on demand, the requirement for robust and versatile optical components has increased. Many of these components, including wavelength division multiplexers, add/drop filters, and switches, make use of a phase shifting array of waveguides, which in this document is called simply a plurality of waveguides, to establish phase differences between propagating signals. When the phase shifted signals are brought together in a coupling or focusing element of the optical device, interference occurs and the multiplexing, filtering or switching function is achieved.
Because the wavelength of signal light is typically in the range of one or two thousand nanometers, the phase shifting plurality of waveguides must be precisely manufactured. This is especially true in systems having very close channel spacing. The manufacturing tolerances in refractive index profiles and geometrical dimensions of typical waveguides frequently are large enough to introduce unacceptable variation in the multiplexing or filtering function of the device.
Therefore most processes for making such devices include a tuning step in which the spectral response, i.e., the signal strength versus wavelength over a preselected band of wavelengths, is adjusted. Typically the tuning step is carried out as an iterative process in which the optical path lengths of the waveguides making up the plurality are changed in order to change the spectral response of the device. The spectral response is measured and another optical path length change made. The process continues until a target spectral response is achieved.
Tuning processes which involve changing the geometry, for example the length, of members of the plurality of waveguides are cumbersome and time consuming. Also, they pose a definite problem in terms of accuracy and repeatability of tuning.
The same is true for tuning processes which rely upon thermal diffusion of dopants, in the members of the plurality of waveguides, to change the respective optical path lengths thereof. An additional time consuming feature of a thermal method is the need to allow a cool down period before a spectral measurement can be made and the next iteration in the tuning carried out. A thermal method is often plagued by hysteresis effects which at best cost time and which at worst can completely undermine the tuning process.
Thus there is a need in the optical waveguide telecommunications industry for an optical interference device tuning method which is fast, stable and repeatable.